This invention relates to a fuel injection system, and more particularly to a fuel injection system for a two-stroke cycle engine.
Recently, a two-stroke cycle engine in which a computer is used for controlling a fuel injection system to keep driving thereof optimum has been extensively used in the art.
A fuel injection system conventionally used for the two-stroke cycle engine generally includes a fuel injection valve or injector provided at a suction passage of the engine, a fuel pomp for feeding the injector with fuel under a constant pressure and a control unit for controlling the injector depending on drive conditions, environmental conditions and the like. The injector includes, for example, a needle valve and an electromagnetic coil for driving the needle valve, wherein the needle valve is rendered open for a period of time during which the coil is kept fed with a drive signal. A pressure of fuel fed from the fuel pump to the injector is kept constant, so that the amount of fuel injected from the injection is proportional to a period of time during which the valve of the injector is kept open or the drive signal is kept fed to the coil. A fuel injection pulse of a rectangular waveform is generally used as the drive signal, so that controlling of a pulse width (injection time) of the pulse permits the amount of fuel injected to be controlled.
The fuel injection system, when it is adapted to be controlled by a computer, generally includes a basic fuel injection time setting means for setting basic fuel injection time Tp using both an engine speed N operated on the basis of on intervals of generation of pulse signals from a signal generator mounted on the engine and a degree of opening of a throttle (hereinafter referred to "throttle opening") .alpha. as a parameter; a fuel injection time setting means for setting fuel injection time Ti by correcting the basic fuel injection time Tp depending on control conditions such as a temperature of cooling water, an atmospheric pressure, a temperature of air sucked into the engine (hereinafter referred to as "suction air temperature") and the like; and an injector drive means for feeding the injector with a fuel injection pulse Pi having a pulse width equal to the fuel injection time Ti set by the fuel injection time setting means.
The basic fuel injection time setting means is adapted to use a three-dimensional map which provides relationships among the engine speed, the throttle opening and the basic fuel injection time which is used as a base for the operation, wherein the three-dimensional map is retrieved while using, as a parameter, the engine speed N and the throttle opening .alpha. detected, resulting in the basic fuel injection time Ti at each of engine speeds being obtained directly or by interpolation from the map.
The fuel injection time setting means carries out the above-described operation for correcting the basic fuel injection time Tp depending on various control conditions including an atmospheric pressure, a suction air temperature, and the like, to thereby provide actual fuel injection time at each of engine speeds. The injector drive means starts to count a predetermined number of clock pulses, which are varied depending on the engine speed, at the time when the signal generator starts to generate a specific signal, to thereby obtain a predetermined injection timing, on the basis of which the injector is fed with the fuel injection pulse of a pulse width equal to the fuel injection time Ti.
The two-stroke cycle engine is constructed so as to feed an air-fuel mixture through a crankcase to a cylinder, so that an air-fuel ratio is affected by a temperature of the crankcase. In particular, when the two-stroke cycle engine is mounted on a vehicle used in a cool district such as a snowmobile, the crankcase is cooled to a very low temperature of -30.degree. C. or less, so that a variation in crankcase temperature is extensively increased. Thus, control of a fuel injection rate depending on only a temperature of cooling water and the like without considering a crankcase temperature causes a failure in appropriate control of an air-fuel ratio, leading to deterioration in startability of the engine, particularly, at a low temperature.
In view of the above, a fuel injection system which is adapted to control of a fuel injection rate depending on a crankcase temperature is proposed, as taught in Japanese Patent Application Laid-Open Publication No. 175121/1991. More specificaliy, the fuel injection system proposed is so constructed that a basic fuel injection pulse width at a low engine speed (hereinafter referred to as "low-speed basic fuel injection pulse width") which is set on the basis of a crankcase temperature is corrected using a correction factor adapted to decrease the pulse width with lapse of time, to thereby set a fuel injection pulse width at a low engine speed (hereinafter referred to as "low-speed fuel injection pulse width") (injection time-at a low engine speed) and a basic fuel injection pulse width at a steady operation (hereinafter referred to as "steady operation basic fuel injection pulse width") is modified depending on various control conditions to set a fuel injection pulse width at a steady operation (hereinafter referred to as "steady-operation fuel injection pulse width"), so that the low-speed fuel injection pulse width and steady-operation fuel injection pulse widths are compared with each other, resulting in larger one of both pulse widths being employed as a pulse width of a fuel injection pulse fed to an injector.
In the proposed fuel injection system thus constructed, the low-speed fuel injection pulse width is set to be larger than the steady-operation fuel injection pulse width at the time when the engine is started. Therefore, a fuel injection pulse having a pulse width equal to the low-speed fuel injection pulse width is fed to the injector at the time of starting of the engine. This causes a fuel feed rate to be increased at the time of starting of the engine, to thereby facilitate starting of the engine. The low-speed fuel injection pulse width is decreased with lapse of time after starting of the engine, to thereby cause the steady-operation fuel injection pulse width to be increased as compared with the low-speed fuel injection pulse width in due course. Thus, the fuel injection pulse of a pulse width equal to the steady-operation fuel injection pulse width is caused to be fed to the injector in a predetermined period of time after starting of the engine or upon-completion of warming-up of the engine resulting in the fuel injection system being shifted to control for steady operation.
The conventional fuel injection system constructed as described above is adapted to determine the low-speed basic fuel injection pulse width based on only the crankcase temperature on the assumption that operation of an accelerator is not carried out at the time of starting and/or warming-up of the engine; so that during the warming-up in which the low-speed fuel injection pulse width is increased as compared with the steady-operation fuel injection pulse width, a fuel injection pulse width is set irrespective of a suction rate of air.
Thus, when the accelerator of the engine is operated at the time of starting and/or warming-up of the engine, an air-fuel ratio is caused to be deviated from an optimum value, resulting in starting of the engine being failed or rotation of the engine being unstable, leading to occurrence of engine stall.